David Cameron has signalled that a number of Britons may have been killed in the hostage crisis in Algeria, but that the total number of UK nationals at risk was "significantly" lower than around 30 as originally feared.

In a statement to the House of Commons, he also indicated that Algerian forces were still pursuing terrorists at the gas plant where gunmen linked to al-Qaida launched an attack early on Wednesday.

He confirmed that a British citizen was one of the people killed in the original assault, but said he was not in a position to give more information about casualties at this stage.

On Thursday night there were reports suggesting that the number of Britons "at risk" was 30. "Thankfully we now know that number has now been quite significantly reduced."

He also confirmed that he had not been given advance notice of the decision by Algerian armed forces to storm the compound on Thursday, even though he had asked his Algerian counterpart Abdelmalek Sellal to be consulted, and that he has stressed the importance of securing the safe release of the hostages.

Cameron said that In Amenas, where the gas field is based, is "one of the most remote places in the world", more than 18 hours by road from Algiers.

"The terrorists first attacked two buses en route to the Amenas airfield before attacking the residential compound and the gas facility at the installation," Cameron said.

"It appears to have been a large, well co-ordinated and heavily armed assault and it is probable that it had been pre-planned. Two of those travelling in the convoy to the airfield were very sadly killed, including one British national, and his family were informed on Wednesday."

Cameron said the attack was organised by Mokhtar Belmokhtar, "a criminal terrorist and smuggler who has been operating in Mali and in the region for a number of years, and who was formerly affiliated with al-Qaida in the Maghreb".

Cameron said it was still not clear exactly how many hostages were taken, but they included Britons and citizens of at least seven other countries.

He said he chaired a meeting of the government's emergency Cobra committee when news of the attack came through and that on Wednesday afternoon he had the first of several conversations with his Algerian counterpart.

"From the outset I have been clear about our implacable opposition to terrorism and said that we will stand with the Algerians in their fight against these terrorist forces," he said.

"But I also emphasised the paramount importance of securing the safety of the hostages. I offered UK technical and intelligence support – including from experts in hostage negotiation and rescue – to help find a successful resolution. And I urged that we and other countries affected should be consulted before any action was taken."

On Thursday morning Algerian armed forces mounted an attack. Cameron said he was not told in advance, but that Sellal spoke to him as the assault was taking place.

"When I spoke again to the Algerian prime minister later last night he told me that this first operation was complete, but this is a large and complex site and they are still pursuing terrorists and possibly some of the hostages in other areas of the site," Cameron said.

"The Algerian prime minister has just told me this morning they are now looking at all possible routes to resolve this crisis."

Cameron went on: "Last night the number of British citizens at risk was less than 30. Thankfully we now know that number has now been quite significantly reduced. And I am sure the House will understand why during an ongoing operation I cannot say more on this at this stage."

Cameron said that BP, which jointly runs the gas plant with the Norwegian company Statoil and the Algerian company Sonatrech, had already evacuated 11 employees from Algeria. He said the government has put aircraft on standby to help with a further evacuation if needed.

Responding to Cameron's statement, the Labour leader Ed Miliband said: "Alongside Algerians and other foreign nationals, those involved are British citizens, seeking to earn an honest living, far from home and their families. It is appalling that innocent, decent people have been targeted in this way. There is no, nor can there ever be, any justification for the taking of hostages."